It seems the dynamic market economy has kicked in with full force.
Not enough people want this jag for 20 million, so the game is forcing sellers to ask for less.
For that you have to wait a bit, this accolade is not meant to be fulfilled in the first two weeks of the game. As time goes by and more rare cars will come up, you will sometimes with some luck have a car that will be sold for 20 000 000.